(Phase) Contrast Transfer Function or CTF is frequently referred to in (HR)TEM. To put it simple, CTF allows to evaluate and compare performances (point-to-point resolution and information limit) of different microscopes.
CTF (Contrast Transfer Function) is the function which modulates the amplitudes and phases of the electron diffraction pattern formed in the back focal plane of the objective lens. It can be represented as:
Clearly, it will be a complicated curve which will depend on:
Cs (the quality of objective lens defined by spherical aberration coefficient)
(wave-length defined by accelerating voltage)
(the defocus value)
k (spatial frequency)
The figure below shows a typical CTF plotted for an imaginary 200 keV microscope:
Important points to notice:
Other important features:
where Ec is the temporal coherency envelope (caused by chromatic aberrations, focal and energy spread , and instabilities in the high tension and objective lens current), and Ea is spatial coherency envelope (caused by the finite incident beam convergence).
Yet more to note: